The fabrication of parabolic mirror surfaces is of interest to optical designers for both large and small telescopes. Typically these surfaces are created by fabricating the best fit spherical surface in glass by traditional grinding and polishing, then hand polishing the workpiece to form the desired aspherical shape. Finally, the glass surface is coated with a reflective material such as aluminum to complete the mirror.
As the need for larger mirror surfaces has grown, the idea of forming segments of a larger mirror, then assembling them into finished parabolic surface was conceived. For a large mirror, the parabolic shape may be made up of 40 or more sections, each of which is approximately one meter or more in diameter. For such an application, each segment may be fabricated by deforming it into a predetermined shape and polishing the deformed shape to a spherical contour. Removal of the bending loads causes the section to return to an unstrained condition in which it formed the desired aspherical shape.
The recent process of optical component fabrication known as diamond turning has dramatically changed the way reflective optical surfaces are now constructed. A diamond turning machine (DTM) typically includes a rotating spindle, and a diamond cutting tool that may be relatively positioned in two axes relative to the workpiece. An example of a typical DTM is the NANOFORM.RTM. 600 made by Rank Pneumo of Keene, N.H. In other words, the cutting tool may be positioned along a radius of the workpiece, as well as positioned in a z-direction or transverse direction into the workpiece to form a predetermined aspherical workpiece surface.
By machining an optical surface with a diamond cutting tool as described, nearly perfect reflective surfaces can be created in a fraction of the time required for the traditional grinding/polishing technology described above. In addition, fabrication is not limited solely to spherical shapes; rather, general aspherical shapes such as parabolas and cones can be fabricated as easily as spheres. Large DTM's have been constructed to fabricate parabolic surfaces as large as nearly two meters in diameter; however, segments of this diameter appear to be approaching the practical limit of such machines.
To facilitate formation of a non-rotational symmetric surface, it has been proposed that the desired surface shape may be modeled with best fit spherical components in combination with non-rotationally symmetric components. See, for example, Gerchman, "A Description of Off-Axis Surfaces For Non-Axisymmetric Surface Generation", submitted for presentation at the ECO3--International Congress on Optical Science & Engineering, The Hague, The Netherlands, SPIE Proceedings, Vol. 1266, Mar. 15, 1990, the entire disclosure of which is incorporated herein by reference.
A theoretical DTM disclosed in a paper by Thompson entitled "Theoretical Tool Movement Required to Diamond Turn an Off-Axis Parabaloid On Axis", SPIE, Vol. 93, pp. 23-29 (1976) includes an actuator that may be moved in the z-direction during a fraction of rotation of the workpiece. Accordingly, a non-rotational symmetric surface may be formed. In other words, off-axis parabolic segments may be fabricated on axis with a fast cutting tool motion in the z-direction and coordinated with the angular orientation of the workpiece.
The relatively fast movement of the cutting tool in the z-direction of a DTM as described in an article by Patterson et al. entitled "Design and Testing of a Fast Tool Servo for Diamond Turning", Precision Engineering, Vol. 7, No. 3, pp. 123-128 (1985) is obtained by a piezoelectric actuator including a series of stacked piezoelectric elements. Unfortunately, most commercially available DTM's do not include a suitable actuator for fast z-direction movement to form a non-rotationally symmetric workpiece surface. Moreover, conventional DTM's include controllers that are not suitable for operating an actuator for the relatively fast movement required.